In brushless DC motor systems, rotor rotation occurs when the phase windings in the stator are energized, creating a magnetic field which interacts with the magnetic field of the rotor. In order to energize the phase windings in the proper sequence, it is necessary to determine the location of the rotor poles relative to the phase windings. When the position of the rotor poles relative to the phase windings is ascertained, the phase windings are energized to produce the appropriate magnetic field. To obtain optimum motor operation, reliable and accurate rotor position information is required.
One method of determining the position of the rotor is to use Hall effect sensors to detect a change in the magnetic field as the rotor poles move past the sensor. Increasing the number of Hall effect sensors will allow for greater resolution. A second method is to monitor the back-EMF generated by the change in the magnetic field in the non-conducting phase winding as the motor rotor rotates. The magnitude and sign of the back-EMF enables the determination of the position of the rotor.
In conventional rotary position detection systems, the electronic components which process the sensor signals are mounted at or near the motor where the operating environment is less than ideal. The electronic components may be mounted remote from the motor, but this requires some form of interconnection to relay position information and control signals between the motor and the controller. This increases the overall cost of the system, and also creates the potential for electrical noise to enter the interconnection. Remote mounted controllers also add to the system cost because of additional wiring, which cost will further increase as sensors are added to improve resolution. Hence, there is a need for a position detection system which may be located remote from the motor and which provides a high degree of noise immunity and a minimum of interconnect wiring between the position sensors and the motor controller.